The present invention relates to a method for detecting a defect in a pixel of an electrical display unit and a method for manufacturing an electrical display unit. More specifically, the present invention relates to: a method for inspecting pixel defects where images are captured from plasma displays or cathode-ray tubes in which defect inspection is made easier and inspection precision is improved, thus allowing quantitative evaluation of these defects; and a method for manufacturing electronic display apparatus that allow easy correction of defects.
Representative examples of electronic displays of the self-emission type include cathode-ray tubes and plasma display panels. These are widely used in television receivers, computer displays, and the like.
Since these displays operate by activating phosphors on a panel, the phosphor must be uniformly applied to the panel during production. If there are defects in the applied phosphor in the production process, defective pixels that do not light up will result. This type of defect will appear as a black dot on a screen display and will greatly decrease the image quality of the display.
Thus, conventionally, before the display is shipped the presence of defective pixels is checked once the phosphor is applied by checking for defective pixels through visual inspection and also by visually checking to see if there are any black dots in the completed display. If a defect is found in the phosphor applying process, all the phosphor is stripped and phosphor is re-applied.
Conventionally, when the number of pixels in electronic displays was not very high, the work involved in visual inspection was not great. However, in current high-resolution displays the number of pixels is on the order of millions of pixels. Thus, defects can be missed due to a lack of experienced inspectors or due to the aging of the inspectors.
Also, the accurate positions of defects cannot be determined since the inspection is performed visually. Thus, it is not possible to take corrective measures by applying phosphor to the defective area. Instead the phosphor has to be re-applied for the entire panel, requiring wasteful effort. Also, with visual inspection, pixel defects cannot be evaluated quantitatively and a detailed report on the defects cannot be provided. Thus, it is not possible to ship out produced displays separately according to quality based on defect status.
Since high-quality products with no defects and high image quality are desired, automated detection and correction of pixel defects in the production process is an important issue. To meet this need, inspection methods performed in the production process of electronic displays use a video camera based on a solid-state imaging elements to capture a display image. Image processing is then performed to provide automated detection of pixel defects.
This method works fine if the number of solid-state imaging elements is sufficiently higher than the number of pixels in the display. However, if there is a higher number of pixels in the display so that the number of solid-state imaging elements is close to the number of pixels on the display, moire (capture moire) patterns will be present in the captured screen.
The moire patterns are caused by insufficient resolution of the imaging elements relative to the display resolution of the display. When the captured image is sampled and processed, the signal components greater than the Nyquist frequency generate aliasing skew.
Thus, conventionally, the acquisition lens is defocused so that signal components greater than the Nyquist frequency are restricted to reduce generation of moire patterns. However, defocusing the lens also reduces the contrast of black dots, and this can reduce inspection precision due to the inability to detect small black dots and the like.
The present invention is provided to overcome the problems described above. The object of the present invention is to provide a pixel defect inspection method wherein, when an image is captured from an electronic display to inspect for defects, inspection precision is improved and pixel defects are quantitatively evaluated by reducing the generation of moire patterns and allowing defects to be easily discovered. Another object of the present invention is to provide a method for manufacturing electronic displays that allows easy correction of display defects and that allows objective evaluation of products.
In order to achieve the above objects, a first configuration of the pixel defect inspection method in electronic display production involves capturing images of the electronic display. Using the captured images, defects occurring in the pixels of the electronic display are inspected. The relative positioning of the electronic display, which is the object being captured, and an imaging element is varied by very small amounts, so that a plurality of image-capture data is obtained. The image data is combined so that image-capture moires generated from the image capture operation are reduced. Pixel defects in the electronic display are detected using the composite image.
With this inspection method, the relative positioning of the object being captured and the imaging element is offset by a fraction of the pixel pitch. This makes it easier to cancel image-capture moire and perform automatic detection of pixel defects. For example, by using an offset of xc2xd pixel, the phase of the image-capture moire is changed by 180 degrees. By adding the offset image and the image without the offset, moire components can be cancelled.
To provide a xc2xd-pixel offset for the imaging element, it would be possible, for example, to have the imaging element fixed to a two-axis table using piezoelectric elements. The imaging element would be moved by very small amounts, thus providing pixel offsets. If the xc2xd-pixel offsets are performed horizontally and vertically, the number of captured pixels is four times the original, thus providing double the resolution compared to not using offsets. This reduces moire and simultaneously increases resolution.
In order to achieve the above objects, a second configuration of the pixel defect inspection method in electronic display production involves capturing images of the electronic display. Using the captured images, defects occurring in the pixels of the electronic display are inspected. Spatially differential processing is performed on the image data obtained from the image capture operation. This allows the pixel defect positions to be determined. Defect contrast, which is defined by the ratio of the luminance of the pixel defect and the luminance surrounding the pixel defect, can be determined to provide a quantitative evaluation of the pixel defect.
With this inspection method, spatially differential processing is performed on the image without moires obtained above. The pixel defects (black dots) are then emphasized and detected, and defect contrast is determined. Additionally, the defect position, size, defect density, and the like can also be calculated automatically.
In order to achieve the objects described above, a configuration of the method for manufacturing electronic displays according to the present invention involves a method for manufacturing electronic displays in which a panel contains phosphor. Phosphor defects are inspected pixel by pixel, and extra phosphor is added to the sections where defects are found.
If the electronic display is a plasma display or a cathode-ray tube, this inspection can be performed after application of the phosphor to determine the positions of the pixels where phosphor is not applied. Thus, a micro-syringe containing phosphor can be positioned to apply corrective phosphor to the defects to eliminate pixel defects.
Also, in an inspection process where a panel is powered and lit, the inspection method described above can be used to determine the position, number, contrast, density, and the like of pixel defects. Based on these, displays can be graded objectively. By affixing a sticker indicating this information, customers can be provided high-quality displays that meet their needs.